Towards Practical Osmotic Energy Capture by a Layer-by-Layer Membrane

Abstract

Recent development in membrane technology for capturing osmotic energy suggests significant performance enhancement due to nanoscale structured materials. A new article in Joule (Chen et al.) presents a bio-inspired layer-by-layer assembled composite membrane of aramid nanofibers and boron nitride nanosheets that demonstrates a major step towards practical osmotic energy harvesting. Recent development in membrane technology for capturing osmotic energy suggests significant performance enhancement due to nanoscale structured materials. A new article in Joule (Chen et al.) presents a bio-inspired layer-by-layer assembled composite membrane of aramid nanofibers and boron nitride nanosheets that demonstrates a major step towards practical osmotic energy harvesting. The ocean is full of mystery, resources, and energy. Oceanic energy, commonly referred to as ‘blue energy’, has attracted significant interest due to its remarkable quantity and varied source of energy, such as waves and tidal, thermal, and salinity gradients. Among these, energy derived from salinity gradients between sea and river water is becoming the key target for harvesting, due to the demand for renewable and clean energy, as well as technological development [1Logan B.E. et al.Membrane-based processes for sustainable power generation using water.Nature. 2012; 488: 313-319Crossref PubMed Scopus (881) Google Scholar]. The amount of energy stemming from salinity gradients is estimated to be 2 terawatts (TW), and the two most promising processes, pressure-retarded osmosis (PRO) and reverse electrodialysis (RED), are continually being developed for this significant harvesting opportunity [2Yip N.Y. et al.Salinity gradients for sustainable energy: primer, progress, and prospects.Environ. Sci. Technol. 2016; 50: 12072-12094Crossref PubMed Scopus (155) Google Scholar]. RED is more attractive due to its relatively simple process (Figure 1A); electricity can be produced just by mixing the seawater and river water. Ever since Pattle reported electricity production by mixing fresh water and salt water in 1954 [3Pattle R.E. Production of electric power by mixing fresh and salt water in the hydroelectric pile.Nature. 1954; 174: 660Crossref Scopus (445) Google Scholar], RED research has developed dramatically due to advancements in technology, especially various tools in nanotechnology [4Siria A. et al.New avenues for the large-scale harvesting of blue energy.Nat. Rev. Chem. 2017; 1: 0091Crossref Scopus (152) Google Scholar]. Osmotic energy harvesting via RED is a broad playground for dealing with fundamental principles and promoting development of this field. Nanofluidic techniques have been introduced into the RED process with remarkable resulting output power density. By employing single-ion-selective nanopores for salinity gradient energy harvesting, researchers achieved a power output ∼26 pW [5Guo W. et al.Energy harvesting with single-ion-selective nanopores: a concentration-gradient-driven nanofluidic power source.Adv. Funct. Mater. 2010; 20: 1339-1344Crossref Scopus (303) Google Scholar] and many more nanofluidic systems were reported thereafter. In 2016, a single-layer 2D MoS2 membrane with nanopores was used for osmotic energy capture, with a large estimated power density up to 106 W/m2, attributed to the atomically thin layer of the membrane [6Feng J. et al.Single-layer MoS2 nanopores as nanopower generators.Nature. 2016; 536: 197-200Crossref PubMed Scopus (455) Google Scholar]. With such a large power density, this seemed to be a promising step towards practical application. Unfortunately, the power density did not scale linearly with the testing area of the membrane. For practical applications, power densities derived from large testing areas will be a key factor moving forward. To explore effective ways of satisfactory energy conversion, learning from nature could be a promising route for material and structural design (e.g., the electric organ of the electric eel [7Zhang Z. et al.Bioinspired smart asymmetric nanochannel membranes.Chem. Soc. Rev. 2018; 47: 322-356Crossref PubMed Google Scholar]). A series of artificial nano-channeled membranes have been developed with significant improvement in energy conversion. Among these materials, organic and inorganic hybrid systems show superior ion transport regulation in the way of layer-on-layer, forming multiple asymmetric structures (e.g., channel sizes and surface charge distribution) [8Xin W. et al.High-performance silk-based hybrid membranes employed for osmotic energy conversion.Nat. Commun. 2019; 10: 3876Crossref PubMed Scopus (84) Google Scholar]. The narrow channels with nanoscale sizes could contribute significantly to ion selectivity and the asymmetric structure could suppress concentration polarization with its ionic diode behavior. Both could contribute to boosting the energy conversion performance of the membrane. Apart from the ion transport regulating properties, the durability of the membrane is another key challenge for practical applications. Recently, a collaborative work from Dan Liu, Nicholas A. Kotov, Weiwei Lei, and colleagues [9Chen C. et al.Bio-inspired nanocomposite membranes for osmotic energy harvesting.Joule. 2019; (Published online December 18, 2019)https://doi.org/10.1016/j.joule.2019.11.010Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar], published in Joule, reports a bio-inspired layer-by-layer (LBL) assembled nanocomposite membrane with excellent mechanical and ionic regulating properties for osmotic energy harvesting (Figure 1B, upper panel). The team chose 1D aramid nanofibers and 2D BN nanosheets as the building blocks for the nanocomposite membrane. Using the idea of learning from nature, they engineered a cartilage-like membrane by employing the 1D fibers as the ‘soft’ layer and 2D nanosheet as the ‘hard’ frame with excellent mechanical properties, chemical stability, and nanoscale structural precision. The aramid fiber building blocks exhibit excellent mechanical properties and are widely used as body, tank, and even ship armor, while the BN nanosheets possess a high chemical stability and good mechanical properties. By combining these two materials with the well-developed and easily accessible LBL technique, a robust aramid-boron nitride membrane is obtained. As a result of the membrane structural engineering, the membrane showed a tensile strength of 370 MPa, higher than a pure aramid membrane (270 MPa). The BN flakes in the composite membrane improved its thermal stability, with an 11°C higher decomposition temperature than that of a pure aramid membrane. The stable and robust membrane showed superior pressure-induced electric streaming current generation and the generated current reached ∼50 nA with 5 kPa pressure in 0.1 M NaCl solution for more than 20 cycles. The membrane could even operate under pressures up to 20 kPa, indicating the potential for membrane operation in more harsh working environments. These render the membrane a major step towards practical application in environments with often harsh conditions. For osmotic-induced energy conversion, the membrane showed good energy harvesting performance, with a power density of 0.6 W/m2 with a 3.14 mm2 testing area. As mentioned above, the testing area can affect the output power density. It is an important step for RED research with large testing areas. With practical application in mind, the membrane is tested to be competent for operating under extreme temperatures (0–90°C) and pHs (3–11). As a promising proposed route for practical blue energy harvesting, there still exist challenges in membrane design and engineering (Figure 1B, lower panel). The ion selectivity of the designed membrane could be improved further, which is a key factor for the energy conversion performance. Also, the membrane structure could be further engineered to facilitate fast ion transport while retaining the excellent mechanical properties. The antifouling aspect of the membrane must also be considered. Besides, the membrane testing area for RED is still a critical issue for evaluating the energy conversion performance. Apart from the RED technique, hybrid systems are also recommended to be tested to evaluate the feasibility of the RED technique [10Mei Y. et al.Recent developments and future perspectives of reverse electrodialysis technology: a review.Desalination. 2018; 425: 156-174Crossref Scopus (183) Google Scholar]. Last but not least, the economics of the membrane will be a major challenge in industrial applications of osmotic energy harvesting. There is still a long way to go for blue energy harvesting; however, this is being revolutionized by bio-inspired innovation and nanotechnology. The work is supported by the National Key R&D Program of China (2017YFA0206900), the National Natural Science Foundation of China (21625303, 21905287, 21988102), and the Strategic Priority Research Program of the Chinese Academy of Science (XDA2010213).

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